The invention relates generally to data communications networks and in particular to a method and apparatus for the detection and regeneration of a lost token in a token passing data communications network.
The advent of intelligent terminals incorporating a microprocessor has lead to the development of new generations of data communications networks. The networks differ from each other in many features, such as for example the topology of the network, the type of apparatus which is attached to the network, and the protocols used on the network to avoid collision of messages or other interferences. In one particular type network, known as a ring network, various nodes are interconnected along a continuous loop so that a message is passed from node to node in a single direction. The message identifies its source and destination in what is generally called the header portion of the message.
In the ring topology, as in many other data communications systems, absent predetermined protocols or priorities, more than one node could access the network simultaneously. A "collision" could then occur wherein two or more messages on the communications media would interfere and hence "garble" each other. As a result, there has been developed what is often called a token-passing protocol wherein a node cannot transmit a message on the network unless it has control or possession of the "token" or marker. According to one protocol, the node receives the token and transmits a data packet appending the token at the end of the data packet. The destination of the packet thus receives the message and passes the token onto a next node (assuming that the destination node does not itself have a data packet to transmit or that the token was not "grabbed" by an intermediate node).
It often happens due to for example noise or other electrical effects that the token can be "lost". This can happen, for example, as the token degenerates and is no longer recognizable due to losses or phase distortions as it passes along the channel. As a result, the network must have a protocol for regenerating the token. There have generally been two methods used for regenerating the token. According to the first method, one and only one of the nodes of the network is assigned the task of regenerating the token. This can often be costly in terms of network performance depending upon the location of the node relative to the location of the "disappearance" of the token and because it provides a single failure point. In other protocols, neighboring nodes generally arbitrate with respect to which node will regenerate the token. This requires complex circuitry and is often quite time consuming and hence reduces network performance.
It is therefore an object of the invention to provide a token regeneration protocol which is inexpensive to implement in terms of both time and hardware. Other objects of the invention are an apparatus and method for regenerating a token in a ring-based data communications network which is reliable, and capable of being implemented in MOS-VSLI integrated circuit designs.